A phylogenetic study of the family Combretaceae with emphasis on the genus Combretum in Africa

Abstract:

The complexity of Combretaceae and lack of information on phylogenetic relationships
within the family led me to explore relationships between and within genera of
Combretaceae by means of combined analyses of plastid (rbcL, psaA-ycf3 spacer, and
psbA-trnH spacer) and nuclear ribosomal ITS sequences for all but three of the 17 genera in
Combretaceae. The current classification of the family into two subfamilies,
Strephonematoideae and Combretoideae, is corroborated. Within Combretoideae, division
into two tribes, Laguncularieae and Combreteae, is strongly supported. Within Combreteae
subtribe Terminaliinae relationships between genera are largely unresolved. Terminalia is
not supported as monophyletic, and two groups were identified, one containing mainly
African species and another of mostly Asian species. Pteleopsis, Buchenavia and
Anogeissus are embedded within Terminalia, and I suggest that all genera of Terminialiinae
with the exception of Conocarpus should be included into an expanded Terminalia. Within
subtribe Combretinae, a clade formed by the two monotypic genera Guiera and
Calycopteris is sister to the rest of the subtribe. Within Combretinae, groupings are
consistent with recent results based on morphological data. Combretum is currently divided
into three subgenera: Apethalanthum, Cacoucia and Combretum. The two last were
included in this study and supported as monophyletic. Meiostemon is sister to subgenus
Combretum, and Quisqualis is embedded within subgenus Cacoucia. I recommend that
subgenus Combretum should be expanded to include Meiostemon and subgenus Cacoucia
to include Quisqualis. The sectional classification within Combretum proposed in earlier
morphological studies is confirmed except for the exclusion of Combretum imberbe from
section Hypocrateropsis into a separate and monotypic section and the inclusion of
Combretum zeyheri (section Spathulipetala) within section Macrostigmatea. In order to
accommodate Combretum imberbe a new section is suggested which is formally described
in Maurin et al. (2010). The reinstatement of previously recognised sections Grandiflora
and Trichopetala, both of which had been sunk into subgenus Cacoucia section Poivrea, is
also suggested. In this study two new species from South Africa are presented: Combretum
nwambiyana O.Maurin, M.Jordaan & A.E.van Wyk and Combretum stylesii Maurin,
Jordaan & A.E.van Wyk. The formal description of these two new species will be done in
regular journals.
Divergence time estimates using a Bayesian MCMC approach implemented in
BEAST suggested a crown date for Combretaceae around 82.6 mya with the two subtribes
Combretinae and Terminaliinae splitting at the end of the Late Cretaceous during a period
marked by a number of mass extinctions both in plants and animals. The Miocene and
Pliocene are characterised by constant speciation with many clades within Combretaceae
succesfully adapting to savanna vegetation and diversifying within it.